The invention relates to a process for producing polyurethane foams, by frothing and drying mixtures of specific polyurethane dispersions and crosslinkers.
In the field of wound management, the use of polyurethane foams as a wound contact layer is well known. The polyurethane foams used for this purpose are generally hydrophilic in order that good absorption of wound fluid may be ensured. Hydrophilic polyurethane foams are obtained by reaction of mixtures of diisocyanates and polyols, or NCO-functional polyurethane prepolymers, with water in the presence of certain catalysts and also (foam) additives. Aromatic diisocyanates are typically used, since they are best foamable. Numerous forms of these processes are known, for example described in U.S. Pat. Nos. 3,978,266, 3,975,567 and EP-A 0 059 048. However, the aforementioned processes have the disadvantage that they require the use of reactive mixtures, containing diisocyanates or corresponding prepolymers, whose handling is technically inconvenient and costly, since appropriate protective measures are necessary for example.
It is also known to produce foams from polyurethane dispersions by incorporating air in the presence of suitable (foam) additives by vigorous stirring. So-called mechanical polyurethane foams are obtained after drying and curing. In connection with wound dressings, such foams are described in EP-A 0 235 949 and
EP-A 0 246 723, the foam either having a self-adherent polymer added to it, or being applied to a film of a self-adherent polymer. The use of the foams as such, i.e. without self-adherent polymers, is not described. In addition, the examples recited in EP-A 0 235 949 and EP-A 0 246 723 mandate the use as crosslinkers of polyaziridines which should now only be used in a limited way because of their toxicity. U.S. Pat. No. 4,655,210 describes the use of the aforementioned mechanical foams for wound dressings having a specific construction of backing, foam and skin contact layer.
The polyurethane dispersions described in EP-A 0 235 949, EP-A 0 246 723 and U.S. Pat. No. 4,655,210 are anionically hydrophilicized through incorporation of certain carboxylic acids such as dimethylol carboxylic acids and neutralization of the carboxylic acids with tertiary amines, for example triethylamine. However, the ammonium carboxylates thus formed are decomposable, in particular at higher temperatures, which sets the amines free again. This is an immense disadvantage in relation to the processing of such products and particularly in skin contact. Furthermore, these polyurethane dispersions were produced using the dimethylol carboxylic acids in dissolved form, for example in dimethylformamide or N-methylpyrrolidone, as a result of which the final products have altogether a high VOC content, 10.8 g litre (without water) in the case of the Witcobond™ 290 H used.
EP 0 760 743 describes such mechanical foams formed on the basis of latex dispersions, but they do not consist of polyurethanes and have worse mechanical properties.
The present invention therefore has for its object to provide novel wound dressings which are based on polyurethanes and are obtainable in a very simple manner and without the use of such building block components or additives as are not generally recognized as safe. It is a further prerequisite that these wound dressings have good mechanical properties, a high uptake capacity for physiological saline and also a high water vapour transmission rate. Moreover, the foams should have a satisfactory water resistance.